The present invention relates to a new and improved arrangement for correcting deviations from the true bearing brought about by specular reflecting surfaces in target tracking radar installations of the type equipped with at least three primary feeds or energy radiating elements.
During the operation of radar installations it is known that apart from the useful signal there are present a multiplicity of undesired echo signals, for instance due to reflection from the ground or water, elevations in terrain, atmospheric and tropospheric layers and so forth. Especially in the case of mobile target tracking radar installations due to their particular concept of intended use and their function of continuously measuring the target, oftentimes changing reflection disturbances occur mostly in all three coordinates and for target speed-determination under different topographical conditions. Depending upon the nature of the target tracking radar installation such also additionally encompasses the function of target detection, so that the radar beam-search movement required for this purpose and accommodated to the terrain automatically arrives at the region of reflecting media and therefore this mode of operation is particularly prone to disturbances.
The suppression of echo signals which emanate from stationary reflectors occurs extensively by means of a so-called moving target indicator with the aid of, for instance, Doppler processing or delay lines and so forth.
On the other hand, suppression of an echo signal is much more difficult in the case of the glancing incidence of a radar beam at a surface designated as a specular reflecting surface which is flat or planar in respect to the wavelength of the transmitted microwave beam. In this case there are formed two wave trains which mutually interfere and at least partially falsify the target echo signal to be processed. The first wave train constitutes the wave directly reflected by the target, whereas the second wave train constitutes a wave reflected by the specular reflecting surface and having a phase shift which is a function of the reflecting media. Hence, at the receiver elements there are formed fields with different field vectors depending upon the momentary relative phase position of the incoming wave trains, these field vectors being additive to one another and influencing the relevant signal removed at the receiver element.
The above observation of the phenomenon known as the specular reflecting effect or mirror effect becomes, however, more significant if the specular reflecting surface is assumed to be a diffuse reflector, such as for instance a moving ocean, irregular rocks or a surface-profiled manoeuvring area. Also the curvature of the earth, depending upon the shape of the lobe-like radiation characteristic of the radar antenna and the elevation of the radar target, especially over specular reflecting water surfaces, has an additional effect upon the falsification of the target echo signal. Of course, the described conditions can be applied also to radar techniques employing different wave shapes in a corresponding modified form and are equally applicable particularly in the case of monopulse radar systems.
In German patent publication No. 2,341,312 there are mentioned numerous methods and apparatuses for suppressing ground echos. As a novel version there is proposed a radar installation embodying an amplifier loop for improving the ratio between the spurious signal to useful signal, the amplifier loop being effective during a receiving time interval between the cycles of two pulses. With the aid of a synchronization device, power divider and coupler a predetermined radiation pattern properly controlled as a function of time is radiated from a transmitter-receiver arrangement. By stepwise switching antenna devices it is intended to influence the receiving direction, and the echo signal emanating from the target as amplified in contrast to the ground echo.
These state-of-the-art radar installations at best afford a specular reflection effect suppression and due to the physical prerequisites can be considered as an optimum solution of conventional techniques. Furthermore, that invention only can be employed with great technological expenditure, also in the environment of target tracking radar installations.
A further improved possibility suitable for target tracking radar installations for the reduction of the specular reflection effect has been published by M. D. Symonds and J. M. Smith in their report entitled "Multi-Frequency Complex-Angle Tracking of Low-Level Targets", IEEE Conference Publication London, October 1973, pp. 166-171. The phase angle which is likewise constant or only varies slowly with constant transmission frequency --the phase angle between the direct signal portion and the reflected signal portion-- is likewise varied by a variable transmitting frequency. Thus, by means of the evaluation of complex phase angles appearing in the echo signals there can be realized a considerable reduction of the deviation from the true bearing of the radar device brought about by the specular reflection effects.
As is however known from the technique of frequency agility rapid changes of the transmission frequency are associated with a relatively large expenditure. Additionally, the present day technology in radar transmitters only permits of the desired large frequency shift needed for obtaining favorable phase differences at the expense of considerable technological difficulties.
All of the heretofore known methods are furthermore associated with the drawback that the measures needed for correction of the reflect effect must be carried out in each successive measurement interval.